In my U.S. Pat. No. 3,398,804, entitled "Method of Drilling a Curved Bore", there is disclosed a method of drilling a curved bore and wherein the desired direction or course is maintained by the relationship established between the bit, reamer and knuckle joint when used in combination with a deflecting tool, such as, a radial whipstock assembly. As discussed in my hereinbefore-referred to patent, the radius of curvature of the curved bore or drainhole necessary to proceed from the original bore to the desired termination point or nadir will determine the radius of curvature to be given the guide surface of the deflecting tool. This, of course, presupposes that the reamer and bit will proceed along the same radius of curvature established by the guide surface. From this radius of curvature, it is possible to determine the angle building capacity of the directional drilling apparatus from the equation ##EQU1## where RC is the radius of curvature, L is measured from the center line of the knuckle joint to the center line of the reamer and a is the distance between the center line of the reamed hole to the center line of the knuckle joint. This relationship still holds true in the drilling of drainholes and assures greater predictability and accuracy in drilling along a predetermined course from a zenith or starting point to the nadir or desired termination point.
In actual practice, it has been found that the angle building capacity of the reamer apparatus may be substantially altered as a result of preferential or increased cutting of the reamer into one portion or wall of the bore. In other words, if the reamer were to cut more deeply into the top of the hole than the bottom it would tend to straighten out the knuckle joint and connected drill string, and the angle building capacity of the tool would be substantially reduced. In the past, little or no attention has been paid to the effect of the reamer design and construction and the important role which the reamer plays in causing the bit to follow a predetermined angle of build. It was assumed that, once the predetermined radius of curvature was established by the guide surface of the deflecting tool, the bit would continuously make a full cut off the bottom of the hole and guide the reamer to uniformly cut the sides of the hole as it follows the bit. In the past, various types of reamers have been employed, such as, blade or spiral-type reamers on the premise that the reamer merely followed the cutting action of the bit along a predetermined radius of curvature in drilling the curved bore. However, it has been found that the reamer will not always follow the radius of curvature established by the guide surface of the deflecting tool but in fact may be influenced by a number of conditions which may alter the actual course of advancement of the reamer as well as the bit. Generally, reamer cuttability will depend to a great extent on rock properties, cutter design, cutter materials, forces or applied loads on the cutter, depth of cut, rotary speed and other factors. Most notably, however, conventional reamers exhibit a tendency to preferentially cut in an upward direction and, as a result, cause a reduction in angle build and an increase in the radius of curvature of the curved bore.
It has been recognized previously that the size of the reamer may vary from that required to cut or ream the bore and prevent stabilization of the bit to a size at which the axial applied load on the apparatus is so great that the reamer is unable to satisfactorily cut. If the reamer does not cut, it operates more as a stabilizer so as to lift the upper joint or drill collar toward the center line of the hole thereby reducing the distance a and increasing the dimension L in the foregoing equation. For this reason, the approach taken in the past has been to make the reamer slightly overgauged and assure that it is capable of cutting substantially along its entire length and particularly at the leading edge or edges of the reamer blades. However, it has now been discovered that a uniform stabilizing action which is concentrated at the leading end is required if the drilling apparatus is to follow a predictable angle of build or radius of curvature and nevertheless retain the ability to uniformly cut or ream the wall or sides of the hole in following the cutting action of the bit. Representative patents on conventional reamer designs are U.S. Pat. Nos. 1,595,922 to O. C. Prindle; 2,131,849 to E. O. Tolson; 2,544,982 to R. Buttolph; 2,589,534 to R. Q. Buttolph; 2,669,428 to J. A. Zublin; 2,687,282 to E. L. Sanders; 2,829,864 to S. R. Knapp; 2,953,350 to S. C. Moore; 3,116,799 to R. Lemons; 3,349,845 to D. R. Holbert et al; and 4,067,404 to G. M. Crase.